Last common ancestor of all animals

- multicellular, heterotrophic

- asymmetry (= no symmetry)

- cellular level of organization (no tissues)

- intracellular digestion (ate bacteria)

- similar to choanoflagellate protists

Sponges

Sponges

Cnidarians

Last common ancestor of the Metazoa

- radial symmetry

- 2 embryonic tissue layers (ectoderm + endoderm)

- extracellular digestion, but with an incomplete gut

Sponges

Cnidarians

Acoelflatworms

Last common ancestor of the Bilateria

- bilateral symmetry

- cephalized (had a head)

- embryos had mesoderm

- still had an incomplete gut

coelomates

Organization of Body Plans: Symmetry

Asymmetrical - sponges- without tissues

Radial symmetry - Cnidarians- 2 embryonic tissues layers (diploblastic)

Bilateral symmetry- 3 embryonic tissue layers (triploblastic)

- with organs

Features of Bilateral SymmetryAnterior (= head end)

Posterior (= tail end)

Ventral (belly, or bottom side)

Dorsal (top side, or back)

rightleft

- only internal cavity is the gut

- space between gut and body wall is tissue-filled

- no fluid-filled space (coelom) to provide rigidity, hence no hydroskeleton for muscles to attach to

Acoelomates

Acoels – the first hunters?

tiny, common marine worms

- no permanent gut, gonads or excretory organs; stomach form temporarily after eating, then cells disband!

- space between temporary gut and body wall is solid (tissue-filled), no coelom

- often live between grains of sand

A true coelom is a fluid-filled cavity enclosed in tissue that develops from mesoderm layer in embryo

- acts as hydrostatic skeleton: point of muscle attachment; gives solidity and strength (so you aren’t just a wet noodle)

Coelomates

Sponges

Cnidarians

Deuterostomes

Protostomes

coelomate animals are divided into 2 major lineages that are distinguishable by features of their embryos:acoel

flatworms

Sponges

Cnidarians

acoelflatworms

Deuterostomes (starfish, us)

Platyhelminthes(flatworms)

Annelidworms

Molluscs

Nematodes

Arthropods

- coelom- complete gut- nephridia (kidneys)

Last common ancestor of coelomates

DeuterostomesEchinoderms

Chordates

ProtostomesPlatyhelminthes(flatworms)

Annelidworms

Molluscs

Nematodes

Arthropods

coelomateancestor

Echinoderms

Chordates

Platyhelminthes(flatworms)

Annelidworms

Molluscs

Nematodes

Arthropods

coelomateancestor

Echinoderms

Chordates

Platyhelminthes(flatworms)

Annelidworms

Molluscs

Nematodes

Arthropods

coelomateancestor

Sponges

Cnidarians

acoelflatworms

Deuterostomes (starfish, us)

Platyhelminthes(flatworms)

Annelidworms

Molluscs

Nematodes

Arthropods

We now recognize two distinct kinds of flatworms:

- primitive acoels (never had a coelom)

- advanced (but simple-looking) Platyhelminthes (lost the ancestral coelom + complete gut)

PlatyhelminthesNemerteans

(ribbon worms)

“false” acoelomates

Although lacking any functional coelom, molecular phylogenetic studies indicate both groups evolved from an ancestor that did have a coelom (because all their relatives have one)

- indicates secondary loss of an ancestral trait, likely an adaptation to their environment and mode of hunting

Phylum Platyhelminthes- Bilateral symmetry (forward movement) but no coelom;

glide on sheet of mucus

- Cephalized: nerves concentrated in cerebral ganglion at front of head, near sense organs

- Triploblastic: mesoderm

gives rise to a muscular

tissue layer in adult body

- Incomplete digestive system

with complex, branched gut

- NO coelom (but ancestor had one)

~20,000species

3 Classes of Platyhelminthes(1) Turbellaria – free-living flatworms

- rely on diffusion of gas across their

thin body wall for respiration

(2) Trematoda – parasites with

2 or more hosts

(3) Cestoda – tapeworms

Fasciola, liver fluke

Taenia

many larval stages in life cycle

Class Turbellaria

Freshwater planaria have a simple,

3-branched gut

Many colorful marine species have

multi-branched guts

Dugesia

Branched digestive systems also used as a circulatory system

Class Turbelaria: Anatomy of a Planarian

Pharynx (muscular eating tube) emerges from middle of posterior side of body

Cerebral ganglion (cluster of nerves) is fed by sensory neurons from eyespots, nerve chords running along either side of body

branched GVC

Class Trematoda: Complex life cycles

Redia, packed full of developing cercaria

2nd, infects fish or crab

1. miracidum2. redia3. cercaria4. metacercaria5. adult

1st infects a snail

Head, w/ hooksfor attachingto intestine wall

Class Cestoda: Tapeworms

chain of sex organs

Intestinal parasites of vertebrates; up to 20 m long

Innovations of Bilaterians, seen in Platyhelmithes

- bilateral symmetry (dorsal - ventral, anterior – posterior axes)

- triploblastic: mesoderm complex organs, muscle tissue

- cephalization: sensory structures concentrated on head, the 1st region to encounter new environments

 Limitations of flatworm body plan:

- rely on diffusion for respiration: must stay wet and thin for O2 to randomly wander in through tissues, and CO2 out

- no coelom = can only move by gliding over surfaces

despite having muscle, there’s nothing for muscles to attach to and work off of (no skeleton)

Movement – 3 kinds of wormsCompare movement in worm phyla with different body plans:

1. flatworms – Planaria

- watch them glide on cilia and a sheet of secreted mucus

2. nematodes (roundworms) – “vinegar worms”

- longitudinal muscles run along the body, but no circular muscles around the middle

- look for distinctive, twitchy movements in lab

3. annelids: earthworms and marine polychaetes

- coelom acts as an internal skeleton: solid bodies

- circular muscles: make one end fat or skinny

- parapodia used as paddle-feet in marine worms